How to count element frequencies in an array

Written by Paul Hudson    @twostraws

If you have an array containing various elements and you want to count how often each item appears, you can do so by combining the map() method with a Dictionary initializer.

First, create an array of items:

let items = ["a", "b", "a", "c"]

Second, convert that to an array of key-value pairs using tuples, where each value is the number 1:

let mappedItems = items.map { ($0, 1) }

Finally, create a Dictionary from that tuple array, asking it to add the 1s together every time it finds a duplicate key:

let counts = Dictionary(mappedItems, uniquingKeysWith: +)

That will create the dictionary ["b": 1, "a": 2, "c": 1] because dictionaries are not stored in order – as you can see, it tells us that “a” appeared twice, while the other two appeared once.

Available from iOS 8.0

Did this solution work for you? Please pass it on!

Tweet

Other people are reading…

• How to use local variable observers
• What is a CGFloat?
• How to format JSON using Codable and pretty printing
• How to remove duplicate items from an array
• How to filter a loop using a where clause

About the Swift Knowledge Base

This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.